Cooking is often referred to as an art, not only because of the combination of ingredients that go into a particular recipe, but also due to the skill necessary for proper application and infusion of varying levels of heat over a given period of time throughout the different phases of the food preparation process. Traditional cookware appliances, such as ovens (microwave ovens being an exception), grills, heat lamps and stoves, all utilize the thermodynamic process of conduction to transfer heat from the outer surface of the food item to its interior. This is generally true regardless of the type of heat source used to heat the surface of the food, be it a radiation heat source (i.e. a heat lamp), conduction heat source (i.e. a stovetop), or a convection heat source (i.e. a convection oven or a food dehydrator).
The time and temperature necessary to cook fully and properly a specific food item through conduction is dependant upon the thermal conductivity of the item, the uncooked temperature of the item (i.e. frozen, room temperature, etc.), as well as the size and shape of the item. A food item having higher thermal conductivity will cook faster than a similarly sized and shaped food item having a lower thermal conductivity, as the heat will more quickly migrate from the outer surface to the interior. Likewise, a generally smaller or thinner food item will cook faster than a generally larger or thicker food item of the same thermal conductivity, as the heat must migrate a shorter distance through the thinner item. Frozen items require considerably more heat to cook than do non-frozen or thawed items. While increasing the cooking temperature for an item will increase the amount of heat that migrates from the surface to the interior of a food item, applying too much heat at one time will result in cooking the outer surface of the item faster than the heat can migrate to the interior, usually resulting in burning or scorching of the surface and undercooking of the interior. Therefore, obtaining real-time information regarding the temperature of the item being cooked, during the cooking process is often beneficial to ensure proper heating.
The use of thermometers or other temperature sensors to monitor and control the cooking process is well known. A common thermometer used to monitor and control the cooking process is a probe-type or contact thermometer which is inserted directly into the food item to obtain a temperature of the interior of the food item. Such thermometers are undesirable for many cooking applications. For, example, when cooking in pots or pans using a lid, the use of a probe-type thermometer requires removal of the lid each time a temperature reading is taken. Continuous removal of the lid during cooking reduces the transfer of heat to the item being cooked, and often results it a detrimental loss of moisture. In addition, the use of contact thermometers usually require manual adjustment of the power of the cooking appliance to obtain and maintain a desired temperature. Not to mention the probe-type thermometer is yet another cooking instrument that must be located and properly used during the often complex cooking process. To overcome the disadvantages associated with contact thermometers, a number of cookware-associated non-contact thermometers have been developed that are attached to, or incorporated into, cookware objects such as pots and pans. Such non-contact thermometers are often in communication with the cooking appliance to control the power level based on the temperature reading. Nevertheless, as discussed below, none of these non-contact thermometers, which control the cooking process solely based upon the temperature of the cookware object, provide a means of obtaining consistent and accurate measurement and control of the temperature of the food item being cooked within the cookware object.
U.S. Pat. No. 3,742,178 to Harnden, Jr. describes a non-contact thermometer placed in thermal contact with an inner wall surface of an inner cup of a cookware object, located between the inner cup and an outer cup in which the inner cup is nested. The inner cup is constructed of a ferromagnetic material that can be heated by an induction coil located in an induction cook-top appliance. Maintaining a stable connection between the temperature sensor and the inner wall of the inner cup is difficult due to thermal expansions and contractions during heating and cooling of the pot. In addition, a large temperature differential may often exist between the inner wall of the inner cup and the outer wall of the inner cup, particularly when extremely cold items are placed within the cookware object while the inner cup is being heated. This large temperature differential makes an accurate determination of the temperature of the food item within the pot difficult, if not impossible to obtain when the temperature reading is taken at the inner wall surface of the inner cup.
In the cookware object taught by Harnden, Jr., the field produced by the induction coil for heating the object also powers the temperature sensor which transmits temperature information to the cook-top appliance via radio frequency to control heating of the cookware object. Although such an arrangement works with induction heating appliances, the temperature sensor of Harnden, Jr. is inoperable when used with a traditional gas or electric stove which heats the cookware object by conduction. Furthermore, the nested cup design of Harnden, Jr., which includes a gap between the inner wall surfaces of the inner and outer cups filled with either thermal insulation material, air or vacuum, is inefficient for conducting heat from the outer cup to the inner cup, making use of the cookware object of Harnden, Jr. with traditional appliances undesirable even if use of the temperature sensor is utilized.
U.S. Pat. No. 5,951,900 to Smrke describes a non-contact temperature sensor that attempts to overcome many of the disadvantages of Harnden, Jr. by inclusion of a temperature sensor mounted to the exterior surface of a lid of cookware object. The temperature sensor of Smrke transmits, either via radio frequency or via wire, temperature information to a cookware appliance to control heating of the cookware object. Although Smrke asserts that a determination of the temperature on the lid of a cookware object is ideal for controlling cooking because such temperature is dependant upon heater power, pot type, food quantity, etc., Smrke does not provide an accurate means of determining temperature of the food item within the cookware object. Furthermore, as discussed above, maintaining a stable connection between the temperature sensor and a surface of the cookware object to which the sensor is attached is difficult due to thermal expansions and contractions during heating and cooling of the object.
Both Harnden, Jr. and Smrke teach cookware objects that are temperature regulated solely by the temperature obtained by the temperature sensors. While temperature information from the object is important, it is often not sufficient to obtain a desired regulation temperature within a desired period of time. For example, it is well known that the power applied to an object placed upon an induction cook-top depends greatly upon the distance between the object's ferromagnetic material and the work coil of the cook-top. Should an object require a particular graduated power application to prevent overheating of some parts of the object while reaching the desired regulation temperature throughout the object, it is essential that the proper power be coupled to the object. Furthermore, most practical heating operations require that the prescribed regulation temperature be reached within a maximum prescribed time. This restraint makes it even more important that proper power be applied during each temperature gradation. A means to correct for inconsistent power coupling that is based upon comparisons between power measurements and stored power coupling data is essential to achieve consistent heating operations and accurate temperature regulation.
U.S. Pat. No. 6,320,169 to Clothier, the disclosure of which is incorporated herein by reference, teaches the use of a Radio Frequency Identification (RFID) tag attached to an induction heatable object to transmit information (typically about a heating characteristic of the object) to a control system of an induction heating device. RFID is an automatic identification technology similar in application to bar code technology, but which uses radio frequency instead of optical signals. RFID systems can be either read-only or read/write. For a read-only system such as Motorola's OMR-705+ reader and IT-254E tag, an RFID system consists of two major components, a reader and a special “tag”. The reader performs several functions, one of which is to produce a low-level radio frequency magnetic field, typically either at 125 kHz or at 13.56 MHz. The RF magnetic field emanates from the reader by means of a transmitting antenna, typically in the form of a coil. A reader may be sold in two separate parts: an RFID coupler, including a radio processing unit and a digital processing unit, and a detachable antenna. An RFID tag also contains an antenna, also typically in the form of a coil, and an integrated circuit (IC). Read/write systems permit two-way communication between the tag and reader/writer, and both the tag and the reader/writer typically include electronic memory for the storing of received information.
Although Clothier discloses that RFID controlled objects can be either cookware or servingware objects, all of the objects disclosed by Clothier are in the form of servingware objects, such as plates and cups. Such objects, which are designed to keep food that has already been cooked at an adequate serving temperature, are subjected to significantly lower temperatures and usually heated for shorter time intervals than are pots, pans and other cookware items, i.e. approximately 250 degrees Fahrenheit for servingware versus approximately 900 degrees Fahrenheit for cookware. Therefore, servingware objects have fewer design constraints than do cookware objects. For example, each of the servingware objects disclosed by Clothier include RFID tags located in the base of the objects, thermally insulated from the heating element or heatable portion of the object. The RFID tag is thermally insulated from the heatable portion of the object due to the limited operating temperatures for most RFID tags. The RFID tag is located in the base of the servingware objects disclosed by Clothier so as to be positioned parallel to and within a range of several inches from the RFID reader/writer located in the induction heating device to enable communication between the tag and the reader/writer during heating of the object. Nevertheless, locating an RFID tag in the base of a cookware object such as a pot or pan, makes adequate thermal insulation difficult to obtain. In addition, even if sufficient thermal insulation is provided, such insulation prevents the cookware object from being heated by traditional cook-top appliances, such as gas or electric stoves conduction stoves as the RFID tag is located directly in the heat-generation zone (i.e. the area directly above the heat source—such as the gas or electric burner for traditional heating appliances, or the induction coil for induction heating appliances—in which the energy used to heat the object is directed) for the object.
The RFID servingware objects disclosed by Clothier are primarily temperature regulated using heating algorithms based upon the heating characteristics transmitted from the object to the induction heating device. Clothier further discloses the inclusion of temperature regulation switches in combination with the RFID tag to better regulate the temperature of the object during heating. The temperature switches disclosed by Clothier operate to prevent or alter the transmission of information from the RFID tag to the induction heating device controller when the thermal switch experiences a predetermined temperature condition. Thus the temperature switches disclosed by Clothier do not provide the ability to obtain a temperature reading other than providing confirmation that the predetermined temperature has been exceeded. This results in a finite number of temperatures, based upon the number of temperature switches, to which the object can be accurately regulated. While such a finite number of predetermined temperatures is acceptable for servingware objects that function to keep already cooked food warm, cookware items, such as pots and pans require a much broader range of regulation temperatures. In fact, cooking of a single item can often require heating in several phases at varying temperatures.
The RFID controlled servingware object combined with temperature switches disclosed by Clothier is in the form of a sizzle plate typically used in restaurants. The temperature switches, which are connected to the RFID tag are placed in contact with the undersurface of the cast iron plate. While such an arrangement may be adequate for lower temperature servingware such as the sizzle plate, the problems associated with maintaining a stable connection to a surface of the heatable object discussed above still exist.